Image amplifier with magnification grid



Feb. 7, 1967 c, J. G. H. WULMS lMAGE AMPLIFIER WITH MAGNIFIGATION GRID Filed NOV. 21, 1963 ll llllllllllllriallllllillllllllllilllilllllrlllll'lviilvbillllllllllll llal ll INVENTOR.

CHRISTIAANJ.G.H.WULMS BY M K A4 United States Patent C 3,303 345 HMAGE AMPLIFEER WETH MAGNIFHCATHQN GRID Qhristiaan .lacohns Gerardus Huhertns Wulms', Emmasingel, Eindhoven, Netherlands, assignor to North American Phiiips Company, Inc, New York, N.Y., a corporation of illeiaware Filed Nov. 21, 1953, Ser. No. 325,27 Claims priority, application Netherlands, Dec. 20, 1962, 286,999 3 Ciaims. (Cl. 250-413) The usual construction of an electron-optical image amplifier comprises a photoelectric cathode, an anode and a fluorescent screen which, in the said sequence are provided in a closed vessel from which the air has been removed. The photo-cathode is accessible from without for radiation which, by the properties of the photo-electric cathode, is converted into electron emission. When using X-rays to obtain an X-ray photograph, an image of a considerably larger brightness can be obtained as a result of the conversion. For that purpose, the electrons emitted by the photo-cathode are focused electron-optically on the fluorescent screen in a manner such that a reduced image of the photo-cathode is formed on the fluorescent screen.

The simplest electron-optical device for this purpose comprises a concave conical photo-electric cathode, a thimble-shaped anode provided at some distance therefrom, in the top of which anode an aperture is provided for passing electrons which move from the cathode to the fluorescent screen provided beyond the anode, and a conductive surface which surrounds the space between the cathode and the anode.

Such a system reproduces an electron-optical reduced image of the photo-cathode on the fluorescent screen. By setting up a voltage between the cathode and the anode, the electrons are accelerated in the direction of the fluorescent screen. The paths of the electrons are under the influence of the equipotential planes of the electric field between the anode and the cathode which are substantially spherical and vary slightly when the voltage varies. The choice of the voltage substantially has no influence on the size of the image and the definition of the image but the brightness of the image is determined by it indeed. The definition of the image may be adjusted by a potential difference between the photo-cathode and the conductive surface.

Image amplifiers are known which comprise an electronoptical system consisting of several electrodes and in which a number of metal cylindrical electrodes is arranged between the anode and the cathode. The electrodes, viewed from the cathode usually have ever smaller diameters and are arranged one behind the other coaxially. The principal object thereof is to avoid charging of the wall when a glass envelope is used. By choosing given voltages for each of the electrodes, it is possible to influence the field distribution between the cathode and the anode and some variation in the size of the image may be produced. The advantage that the size of the image is not only determined by the geometry of the optical system but also by the electrode voltages is outweigher by the increase of the difficulties which are experienced during the manufacture of such a system which, as a result, is

considerably more expensive. In addition, the use of a number of electric voltages which each influence the adjustment of the image is not attractive because the voltages must accurately retain the adjusted values. Consequently, for each voltage additional means are required to prevent any voltage variations.

It is known that in an image amplifier which is provided with a photoelectric cathode and an anode, a conductive surface surrounding the space between the cathode and the anode so that the wall of the envelope is protected from loading, a small voltage between the conductive surface and the cathode is suflicient to sharply focus the image. It has already been noted that this auxiliary voltage is used to correct the lack of definition which occurs if the distance between the cathode and the anode does not exactly correspond to that which is required without this potential difference for the formation of a sharp image on the fluorescent screen. It has appeared from a further investigation that with such an electrode system an image is obtained beyond the anode which is sharp over a distance between narrow limits. When the distance between the anode and the fluorescent screen increases, a smaller part of the cathode is reproduced, per surface unit, on the picture screen. By varying the position of the fluorescent screen, the image magnification could be controlled somewhat. in this case difliculties are experienced as a result of which a useful construction is particularly complicated.

The object of the invention is to vary in a simple manner the size of the image, it being possible to choose between two fixed values of the magnification when using an image amplifier the electrode system of which consists of a spherical cathode, an anode and a conductive surface which surrounds the space between the anode and the cathode. According to the invention, in the electron accelerating space at a distance from the anode which is small compared with the distance between the anode and the cathode, an electrostatic screen is provided having an aperture the center of which lies on the optical axis. The screen takes the place of an equipotential plane determined by the potential distribution between the anode and the cathode. The screen has a shape corresponding to and the same voltage as either that of the equipotential plane or the anode voltage.

Another feature of the invention is that in a device which is provided with such an image amplifier and a direct voltage source with connections for a high and an intermediate voltage, the latter is determined by the potential of the equipotential plane which coincides with the curved screen in the image amplifier if between the anode and the cathode the high voltage is operative, a switch effecting the connection of the intermediate voltage or the connection of the high voltage to the screen.

The invention will be described with reference to the accompanying drawing in which the sole figure shows an image amplifier, in section in a plane through the axis thereof, according to the invention.

The wall of the amplifier housing consists of glass and comprises a cylindrical part 1 with a spherical closure 2 at the one end. The other end is closed by a somewhat curved bottom 3. It is not necessary that all these parts be manufactured from glass. The cylinder jacket 1 may consist of metal. Such a wall renders the use of a focusing electrode 4, which, in the figure, is provided inside the cylindrical wall, superfluous. If the image amplifier is intended for X-ray investigation, the spherical end wall 2 may be made from metal which only absorbs X-rays slightly, preferably aluminum. The cylinder wall 1 must be connected to it in an air-tight manner but it must also be suitable for connecting the glass bottom 3 by sealing, at least to provide glass parts in an air-tight manner which are required for the sufficient insulation of current conductors which have a high electric voltage with respect to the metal wall. Such a voltage is supplied to the anode 5. The anode consists of a cylinder 6, a conical part 7 and a spherical top 8 which are all made from metal. The cylindrical part 6 is clamped around a reentrant part 9 and the top of the anode includes an aperture 10. The current supply wire 11 connects the cylinder 6 to a conductor 12 through the wall 13 which encloses the re-entrant part 9. This wall part 13 is a spout at the bottom 3. The re-entrant part is closed by a transverse wall 14. On the inside of the transverse wall 14 in a field-free space a fluorescent screen 15 is located consisting of a thin layer of fluorescent material on a transparent carrier, for example mica or glass. The carrier may also be omitted and the transverse wall 14 coated with a thin layer of fluorescent material. In this case the surface of the transverse wall must be entirely flat and smooth so that it may be more advantageous to manufacture the fluorescent screen separately.

To the arched front part 2 of the jacket 1 suspension wires 16 are sealed in the glass and connected to the focusing electrode 4. One wire service as a current conductor for supplying voltage to the electrode 4. A wire 17 also sealed in the wall section 2 serves as a current conductor and is connected to the photo-electric cathode 18.

Photo-electric cathodes for image amplifiers which are used for X-ray tests and such in image amplifiers for other purposes, which are termed luminoscopes, are well-known. The dish 18 shown in the drawing represents the one or the other photo-cathode. The connection is effected by an insulating edge 19 which connects the dish 18 along the circumference to an inwardly bent edge 20 of the focusing electrode 4. This electrode 4 also at the other end comprises an inwardly bent edge 21 which part also serves for checking the charging of the glass wall.

If by means of a suitable voltage source 22, of which the positive terminal 23 is connected to the wire 12 and the negative terminal 24 to the wire 25, the anode is brought at a sufficiently high positive potential with respect to the cathode 18 (a suitable voltage is 25 kv.), the electrons emerging from any point of the cathode surface are conducted to the anode in the form of a beam. In case of true focusing, these electrons impinge upon the fluorescent screen 15 at the same point which is the image point of the point of the cathode 13 from which the electron beam has started. In this manner each emitting point of the cathode finds an image point on the luminescent screen. The electron beams intersect in the proximity of the aperture in the anode 5.

The sliding contact 27 of a potentiometer resistor 28 which is connected between the positive and the negative terminals 23 and 24 of the current source 22 is connected to the focusing electrode 4 by the wire 26. The potential of this electrode is controllable with respect to the cathode 18 and is adjustable, for example, between 0 and 200 volts and serves for focusing the greatest definition with which the image appears on the luminescent screen. For correct focusing, an image is produced on the fluorescent screen in which each part corresponds to a portion of the image on the surface of the photocathode.

Details of the image which are of importance for the radiologist are often not sufficiently clearly reproduced, even if, as usual, the fluorescent screen is viewed through a magnifying lens because the detail definition only permits a restricted light-optical magnification. The somewhat unsatisfactory condition which is the result of this is hardly improved if the image amplifier is replaced by another amplifier producing another image size because the replacement of the tube and the refocusing of the image is too time-consuming. In addition, the manufacture of such image amplifiers is too expensive to store tubes having different factors of magnification.

If in such an image amplifier a conductive surface formed according to a given equipotential plane is provided with an aperture for passing the electrons and connected to a voltage which has no influence on the potential distribution between the anode and the cathode, the reproducing power of the electrode system experiences no variation. Such a conductive surface is the curved screen 29 provided in the proximity of the anode 5. The aperture St} in this screen is chosen to be larger than the aperture 15) in the top of the anode. The screen 29 is supported by a number of supporting rods 31 which are sealed in the bottom 3 and one of which is connected to the switching arm 33 of a switch by a wire 32. When the amplifier is in normal use, the screen 29 is non-operative. In this case it is connected through the switching arm 33 to a point 34 of the potentiometer resistor 28, the potential of which is the same as the potential of the equipotential plane in the position where the screen 29 is. By switching the switching arm 33 to the positive terminal 23 of the current source 22, the potential of the screen 29 is increased to the anode voltage. As a result of this the size of the image which is formed on the fluorescent screen of a part of the cathode varies. The reduction factor decreases and the image becomes larger. In addition to the size of the image, the field strength on the cathode increases as a result of the fact that the anode is provided nearer to the photocathode. A higher field strength is favorable since as a result of this the chromatic aberration decreases so that, consequently, by using the invention, the photo-cathode image is observed not only larger but also with a better detail definition.

For varying the magnification by a factor 2, the screen is approximately in the position of the potential plane which corresponds to 10 kv. The distance between the anode top 8 and the screen 29 will be somewhat less than the radius of the anode sphere. From calculations of accurately spherically-symmetric systems of cathode and anode, the position and the potential of the screen can readily be derived, although the exact shape and the arrangement of the screen can only be determined experimentally.

The object of the annular fold 35 in the bottom 3 is to increase the insulation length between the supporting wires 31 and the part of the wall where the distance to the focusing electrode 4 is small.

What is claimed is:

1. An electron optical image amplifier comprising an envelope, a concave spherical photocathode for producing an electron image corresponding to an optical image, a thimble-shaped anode within said envelope spaced from said photocathode, said thimble-shaped anode having an aperture therein for the passage of electrons emanating from the photocathode resulting from light incident thereon, a fluorescent screen positioned to receive electrons passing through the aperture in the anode for converting the electron image into a visible image, a conductive wall surrounding the space between the photocathode and the anode, means to apply potentials to said photocathode, anode and conductive wall whereby electrons produced by the photocathode are focussed at the fluorescent screen, an electrostatic screen positioned between and closer to the anode than the photocathode, said electrostatic screen having an aperture therein the center of which lies on an optical axis extending between said photocathode and said fluorescent screen, said electrostatic screen being coincident with an equipotential plane determined by the potential distribution between the photocathode and the anode, said screen having a shape corresponding to that of said equipotential plane, and means to apply a potential to said electrostatic screen equal to the anode potential whereby a magnified image is produced on the fluorescent screen in which each portion corresponds to a portion of the image on the photocathode.

2. An electron-optical image amplifier as claimed in claim 1 in which the means to apply a potential to the electrostatic screen includes switch means to connect the electrostatic screen to a potential equal to the potential of the equipotential plane at the position of the screen 3. An electron-optical image amplifier as claimed in claim 1 in which the conductive wall surrounding the 6 space between the photocathode and the anode constitutes a portion of the envelope.

References Cited by the Examiner UNITED STATES PATENTS 2,700,116 1/1955 Sheldon 250-213 2,702,355 2/1955 Felici 250-213 2,757,293 7/1956 Teves et al. 31365 10 RALPH G. NILSON, Primary Examiner.

M. ABRAMSON, Assistant Examiner. 

1. AN ELECTRON-OPTICAL IMAGE AMPLIFIER COMPRISING AN ENVELOPE, A CONCAVE SPHERICAL PHOTOCATHODE FOR PRODUCING AN ELECTRON IMAGE CORRESPONDING TO AN OPTICAL IMAGE, A THIMBLE-SHAPED ANODE WITHIN SAID ENVELOPE SPACED FROM SAID PHOTOCATHODE, SAID THIMBLE-SHAPED ANODE HAVING AN APERTURE THEREIN FOR THE PASSAGE OF ELECTRONS EMANATING FROM THE PHOTOCATHODE RESULTING FROM LIGHT INCIDENT THEREON, A FLUORESCENT SCREEN POSITIONED TO RECEIVE ELECTRONS PASSING THROUGH THE APERTURE IN THE ANODE FOR CONVERTING THE ELECTRON IMAGE INTO A VISIBLE IMAGE, A CONDUCTIVE WALL SURROUNDING THE SPACE BETWEEN THE PHOTOCATHODE AND THE ANODE, MEANS TO APPLY POTENTIALS TO SAID PHOTOCATHODE, ANODE AND CONDUCTIVE WALL WHEREBY ELECTRONS PRODUCED BY THE PHOTOCATHODE ARE FOCUSSED AT THE FLUORESCENT SCREEN, AN ELECTROSTATIC SCREEN POSITIONED BETWEEN AND CLOSER TO THE ANODE THAN THE PHOTOCATHODE, SAID ELECTROSTATIC SCREEN HAVING AN APERTURE THEREIN THE CENTER OF WHICH LIES ON AN OPTICAL AXIS EXTENDING BETWEEN SAID PHOTOCATHODE AND SAID FLUORESCENT SCREEN, SAID ELECTROSTATIC SCREEN BEING COINCIDENT WITH AN EQUIPOTENTIAL PLANE DETERMINED BY THE POTENTIAL DISTRIBUTION BETWEEN THE PHOTOCATHODE AND THE ANODE, SAID SCREEN HAVING A SHAPE CORRESPONDING TO THAT OF SAID EQUIPOTENTIAL PLANE, AND MEANS TO APPLY A POTENTIAL TO SAID ELECTROSTATIC SCREEN EQUAL TO THE ANODE POTENTIAL WHEREBY A MAGNIFIED IMAGE IS PRODUCED ON THE FLUORESCENT SCREEN IN WHICH EACH PORTION CORRESPONDS TO A PORTION OF THE IMAGE ON THE PHOTOCATHODE. 